My observations on why and why not to use some pragmas

#!/usr/bin/perl -T
use strict;
use warnings;
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strict also does some runtime checking of your code and as such probably should be left in for production.

use diagnostics loads up a big overhead on startup and probably should only be for debugging due to the performance hit.

Also one should put -T on the #! line of any program which takes user input to detect a host of other insecure coding practices.

warnings dont seem to pose any noticeable delay and if your code emits no warnings then its probably easier to maintain that code that dumps a bunch of them.

Finally, strict and warnings on and silent will earn you points with the person who inherits the code base from you to maintain.